All of creation knows of the existence of God. Paul labors this point in his letter to the church in Rome when he states that "since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made" (Romans 1:20). Every man who has rejected the existence of God throughout the centuries knows this truth, but as Paul said, they "suppress the truth in unrighteousness" (Romans 1:18). Every atheist knows that the LORD reigns on high, but because of their hatred for Him they deny Him, and so when witnessing to an unbeliever one should always come with a presupposition knowing that in the heart of the unbeliever he fully believes in the full existence of God.
To come with a pessimistic mindset thinking that one must first prove the existence of God to an unbeliever before one can preach the gospel is a false assumption. The use of logic, however, can be an useful tool in the practice of apologetics. Some devout classical apologists presuppose that pre-suppositional apologists are anti-logic, which is a false presupposition on their part. I, being a pre-suppositionalist myself, actually engage in the use of logic often while witnessing to unbelievers. When people ask questions, us as Christians must have answers, as Paul writes, "We are destroying speculations and every lofty thing raised up against the knowledge of God" (1 Corinthians 10:5). The use of logic can be a tool of much worth, but for it to be the main tool of apologetics is when the danger arises.
The reason I am against adherring to the practice of classical apologetics is because it is trying to argue a worthless argument. The existence of God, as Paul stated, "has been clearly seen" (Romans 1:20) to all mankind. The argument for the existence of God has been won, and what is needed is not primarily an argument, but the conversion of the unbeliever. Paul said that "the natural man does not accept the things of the Spirit of God, for they are foolishness to him; and he cannot understand them, because they are spiritually appraised" (1 Corinthians 2:14). Unless a man is regenerated he will not come to faith, for it is impossible for him to do so. The apologetics of a believer are to rest on the power of God and not on the wisdom and intellect of man. To base one's arguments off of logic is like fighting a dead man, you may win the fight, but he is still dead. The unbeliever will not be converted through the argument of logic, but rather the Gospel. The preaching of the Christian is to teach a faith that does not rest on the wisdom of men but on the power of God. The Spirit of God unblinds the eyes of His elect through the preaching of the Word, not the arguing of logic.
"And when I came to you, brethren, I did not come with superiority of speech or of wisdom, proclaiming to you the testimony of God. For I determined to know nothing among you except Jesus Christ, and Him crucified. I was with you in weakness and in fear and in much trembling, and my message and my preaching were not in persuasive words of wisdom, but in demonstration of the Spirit and of power, so that your faith would not rest on the wisdom of men, but on the power of God."
[1 Corinthians 2:1-5]